An important aim of ongoing research in the semiconductor industry is increasing semiconductor performance while decreasing power consumption in semiconductor devices. Planar transistors, such as metal oxide semiconductor field effect transistors (MOSFET) are particularly well suited for use in high-density integrated circuits. As the size of MOSFET and other devices decrease, the dimensions of source/drain regions, channel regions, and gate electrodes of the devices, also decrease.
Strained silicon transistors provide increased semiconductor performance with decreased power consumption. Strained silicon transistors are created by depositing a layer of silicon germanium (SiGe) on a bulk silicon wafer. A thin layer of silicon is subsequently deposited on the SiGe layer. The distance between atoms in a SiGe crystal lattice is greater than the distance between atoms in an ordinary silicon crystal lattice. There is a natural tendency of atoms inside different types of crystals to align with one another where one crystal is formed on another crystal. As such, when a crystal lattice of silicon if formed on top of a layer of SiGe, the atoms in the silicon crystal lattice stretch or “strain” to align with atoms in the SiGe lattice. A resulting advantage of such feature is that the strained silicon experiences less resistance to electron flow and produces gains of up to 80% in speed as compared to ordinary crystalline silicon.
Shallow trench isolation (STI) provides another technique to shrink device size. The use of STI significantly shrinks the area needed to isolate transistors better than local oxidation of silicon (LOCOS). STI also provides superior latch-up immunity, smaller channel width encroachment, and better planarity. The use of STI techniques eliminates the bird's-beak frequently encountered with LOCOS.
Strained silicon layers are typically epitaxial layers formed by chemical vapor deposition (CVD) to a thickness of about 100 Å to about 300 Å. The thickness of the strained silicon layer depends on the Ge concentration in the SiGe layer. The critical thickness of a strained silicon layer is the maximum thickness below which the strained silicon is defect free. At thicknesses above the critical thickness, the strained silicon layer tends to relax, the crystalline geometry of the relaxed region becoming more like ordinary crystalline silicon and less like a SiGe crystal. When the Ge concentration in the SiGe layer is about 15%, the critical thickness of the strained silicon layer is about 300 Å. When the Ge concentration in the SiGe layer is about 20%, the critical thickness of the strained silicon layer is about 200 Å. When the Ge concentration in the SiGe layer is about 30%, the critical thickness of the strained silicon layer is about 100 Å.
Strained silicon layers also tend to relax in the portion of a strained silicon layer adjacent to the boundary of a strained silicon layer and an STI region trench sidewall. A semiconductor device 50 as shown in FIG. 1, comprises a strained silicon layer 16 formed overlying a SiGe layer 14 on a silicon-containing substrate 12. An STI region 48 with a trench sidewall 52 borders the strained silicon layer 16 and the SiGe layer 14. A gate oxide layer 36 and polysilicon gate electrode layer 38 are formed overlying the strained silicon layer 16. The portion of the strained silicon layer 42 adjacent STI region trench sidewall 52 tends to relax, becoming more like ordinary crystalline silicon. As a result of strained silicon relaxation, electrons move slower through the portion of the strained silicon region adjacent a STI region 42 than through the remaining portion of the strained silicon layer not adjacent to the STI region 46.
The term semiconductor devices, as used herein, is not to be limited to the specifically disclosed embodiments. Semiconductor devices, as used herein, include a wide variety of electronic devices including flip chips, flip chip/package assemblies, transistors, capacitors, microprocessors, random access memories, etc. In general, semiconductor devices refer to any electrical device comprising semiconductors.